Cochlear implants have proven to be clinically effective in providing the sensation of hearing to individuals with a profound hearing loss. Such devices typically comprise an array of implantable electrodes located within the cochlea of the individual which directly apply electrical stimulation to the surviving auditory neural elements which are perceived by the brain as sounds. The electrical stimulation applied to the auditory pathway of the individual is derived from an acoustic signal which is processed by a sound processor according to a variety of sound processing strategies.
Traditionally, most early sound processing strategies have concentrated on processing the acoustic signal in a manner that enables the recipient to obtain some degree of open-set speech discrimination. As such, existing sound processing strategies have been successful in enabling the recipient to understand conversation speech in quiet surroundings without the aid of lip reading. However, studies comparing normal hearing to cochlear implant listening have shown that voice pitch is poorly perceived by users of cochlear implants. In addition, pitch information which forms the basis for melody in music has also been shown to be poorly perceived by users of cochlear implants.
Voice pitch information can play an important role in speech perception. Voice pitch information can provide important cues to linguistic features of speech, such as intonation to assist a listener in determining the contrast between a question or a statement being made by a speaker, as well as any emphasis that may be placed on a word or words by the speaker. Voice pitch information can also provide important cues to paralinguistic features of speech, to aid in speaker identification and the determination of the emotional state of the speaker, as well as assisting the listener to segregate concurrent speakers.
Most importantly, it bis been established that voice pitch information is crucial for perception of tonal languages, such as Mandarin and Cantonese, where a change in fundamental voicing frequency within the same phonemic segment causes a change in lexical moaning.
Various sound processing strategies have been developed for processing of sound signals for use in stimulating auditory prostheses, such as cochlear implants. One such strategy, referred to as a “multi-peak strategy”, focuses particularly on coding of aspects of speech, such as formants and the fundamental voicing frequency (F0). For this strategy, voice pitch information has been predominantly coded by way of the electrical stimulation rate. However, whilst results with this strategy have shown that pitch could be perceived, performance deteriorates rapidly in real world situations, especially in the presence of noise. Other strategies have been proposed that code voice-pitch information (for voicing frequencies up to approximately 300 Hz) by way of amplitude modulation, at a frequency equal to or related, to the voicing frequency, in the envelope of the electrical stimulus signals. These strategies include the Spectral Maxima Sound Processor (SMSP) strategy (which is described in greater detail in Australian Patent No. 657959 and U.S. Pat. No. 5,597,380 by McDermott & Vandali, 1991), and more recent implementations of this strategy known as the Spectral Peak (SPEAK) strategy (Skinner et al., 1994; Whitford et al., 1995), and the Advanced Combinational Encoder (ACE) strategy (Vandali et al., 2000; Skinner et al., 2002). However, studies examining pitch perception with these strategies have shown that the salience and accuracy or cues to pitch can be poor for some signals and in real world situations and performance deteriorates rapidly in noise.
A number of modifications to existing sound coding strategies have been proposed in an attempt to improve coding of voice and/or musical pitch. These include the Modulation Depth Enhancement (MDE) and Multi-channel Envelope Modulation (MEM) strategies described in Vandali et al., 2005 and disclosed in US Patent Publication No. 20060080087; Vandali, A. E., and van Hoesel, R. J. “Modulation depth enhancement for tone perception,” U.S. Pat. No. 7,561,709; and Vandali, A. E., van Hoesel, R. J., and Seligman, P. M. “Pitch perception in an auditory prosthesis,” US continuation-in-part patent application of US patent application US 2006/0080087. In addition, McDermott and McKay proposed a device for improving coding of pitch in cochlear implant systems (McDermott, H., and McKay, C. “Cochlear Implant Sound Processing Method and System”, US patent application US 2005/0107843. Each of these documents is incorporated herein by reference.
Despite the above attempts to improve the coding of voice and/or musical pitch particularly for use in stimulating auditory prosthesis, there is still a need to improve such perception in a range of hearing situations. As such, the present invention addresses this need by creating a complete system that codes voice and/or musical pitch information in a cochlear implant system in an effective manner which is robust to the effects of competing noise and/or interfering signals.
The above references to and descriptions of prior proposals or products are not intended to be, and are not to be construed as, statements or admissions of common general knowledge in the art. In particular, the following prior art discussion does not relate to what is commonly or well known by the person skilled in the art, but assists in the understanding of the inventive step of the present invention of which the identification of pertinent prior art proposals is but one part.